The invention relates to a knee-joint endoprosthesis with a tibial part having a flat tibial bearing surface, a bearing body that can be displaced thereon in the anterior-posterior direction and comprises two concave bearing shells within which a femoral joint part can be movably supported, and a rotational guide means that permits rotation of the bearing body on the tibial bearing surface about an axis perpendicular to the latter.
A knee-joint endoprosthesis of this kind is known, for example, from the patents EP 0 529 408 B1 or EP 0 519 873 B1.
Both of these known constructions represent knee-joint endoprostheses that comprise not only joint-surface congruence and the translational mobility of a so-called meniscus knee, but also preserve these properties even during rotational movements and to a great extent ensure protection against luxation of the bearing body and enable movements to occur in a nearly normal, physiological manner.
The construction according to EP 0 519 873 B1 is characterized by the fact that the translational movement of the bearing body is not limited by stopping devices.
However, it has been found that soft tissues can then very easily become irritated and the cruciate ligaments can be overloaded.
Furthermore, it should be possible for the surgeon to wait until the closing phase of an operation to decide whether the knee implant employed should permit both translation and rotation or only rotation of the bearing body. With the known constructions, the surgeon does not have this choice.
Finally, the surgeon should have the opportunity during the closing phase of the operation to determine the maximal translational movement of the bearing body in the posterior-anterior direction.
Accordingly, it is the objective of the present invention to create a knee-joint endoprosthesis of the kind described above that avoids the above-mentioned disadvantages of the known constructions, provides maximal degrees of freedom for guidance suitable to the anatomy, and does not irritate soft parts or overload the cruciate ligaments. Furthermore, this construction should largely avoid excessive complexity in the sliding bearing between bearing body and tibial bearing surface, and should operate without bending moments.
This objective is achieved by the characterizing features given in claim 1. That is, the knee-joint endoprosthesis in accordance with the invention is characterized in that the rotational guide means for the bearing body comprises a steering rod that is mounted so as to be pivotable relative to the bearing body and at the tibial part is guided in a medial/lateral direction and/or rotationally.
Structural details of the knee-joint endoprosthesis in accordance with the invention are described in the subordinate claims.
In one advantageous embodiment, at the tibial part the steering rod is guided medially and laterally within an aperture in the form of slot that extends in the anterior-posterior direction. At the bearing body, in contrast, it is either mounted so as to be rotatable and pivotable relative thereto or it is so mounted that together with the aperture in the tibial part (i.e., the component incorporating the aperture) it can be rotated about an axis perpendicular to the tibial bearing surface.
Another preferred embodiment is characterized by the fact that the slot at the level of the tibial bearing surface delimits a recess in the form of a pocket hole, which is situated within a distally extending tibial housing and expands in a trapezoidal form, at least in its upper region, from distal to proximal in the anterior-posterior direction. In another preferred development the recess is also trapezoidally expanded towards its floor, so that the steering rod can be pivoted with no bending moment about an axis of rotation spaced apart from its lower end (situated at the level of the narrowest part of the recess).
Medially and laterally the pocket hole is delimited by walls that extend parallel to one another and approximately perpendicular to the tibial bearing surface, which are spaced apart from one another by a distance slightly greater than the diameter of the steering rod, or section thereof, that is contained within the pocket hole. The rod or rod section should be guided so that it can slide with medial and lateral clearance.
The pivot bearing of the steering rod in accordance with the invention in the anterior-posterior direction is achieved in a variant by providing the proximal end of the rod with a spherical head that inserts into a complementarily formed pivot-bearing recess in the bearing body; in particular, the head can be locked therein. Alternatively, at the proximal end of the steering rod there can be provided a pivotal axis that extends approximately parallel to the tibial bearing surface in the medial/lateral direction, which likewise engages a complementary pivot-bearing recess in the bearing housing.
The sliding bearing of the steering rod at the tibial part is advantageously implemented within a bearing sleeve made of plastic, in particular polyethylene, and positioned at the tibial part so as to be secured against rotation, in particular is disposed within a tibial housing that extends distally. This arrangement ensures that after the operation the slideway of the steering rod remains exactly in the posterior-anterior direction.
In a special embodiment the slot-like recess is curved so as to be concave towards the medial side, i.e. convex towards the lateral side. In particular, it has the shape of an arc of a circle, with a center of curvature situated medially, in particular within the contour of the bearing body. The steering rod or section thereof (steering shaft) that interacts with this recess preferably has a cross-sectional shape that is either circular or comprises two curved wall sections that match the curved shape of the recess.
Otherwise, the steering rod or the section thereof that passes is contained in the recess is shaped to suit a recess with flat wall sections; that is, it is either circular in cross section or its cross section comprises two lateral semicircles connected by straight lines.
In an alternative embodiment the steering rod is pivotably seated by its distal end at the tibial part, such that this distal and/or the proximal pivot bearing at the bearing body permits a translational movement along the axis of the rod; thus a permanent sliding contact between the bearing body and the tibial bearing surface is guaranteed, regardless of the angular position of the steering rod.
The steering rod preferably comprises a body-compatible metal alloy (titanium alloy), as does the tibial part.
In the following, embodiments of a knee-joint endoprosthesis in accordance with the invention are described in greater detail with reference to the attached drawings, wherein